The present invention relates to a sputter ion pump that may be used for evacuating a space through which electron beams pass for example in an electron microscopy or an accelerator.
It is known that such a sputter ion pump comprises an anode electrode and a cathode electrode arranged within a vacuum chamber. High voltage is applied between the anode and cathode electrodes so that residual gas molecules to be evacuated are collided with electrons that are spirally moving by means of electromagnetic field, and thus are ionized. The cathode electrode is subjected to a sputtering by means of the ionized molecules to activate the surfaces thereof. Gas molecules are adsorbed on or embedded in the activated surfaces of the cathode electrode, or gas molecules are caught by the surfaces of the anode electrode, thereby performing an evacuation of gases.
One example of such conventional sputter ion pumps is disclosed in Japanese UM Publication No. 3-48838. The disclosed ion pump is applied for an electron microscopy in that two annular magnets are provided on a yoke member in such a manner that ion adsorption cells having an anode function are arranged between the annular magnets. Magnetic pole pieces are arranged on a magnetic circuit of a leakage magnetic flux of the annular magnets so that almost all of the leakage magnetic flux in a direction of a central aids pass through the magnetic pole piece thereby capable of concentrating the leakage magnetic flux.
Japanese Patent Publication No. 7-59943 discloses another conventional sputter ion pump in that two annular cathode electrodes are oppositely arranged between which an annular anode electrode is sandwiched, the annular anode electrode comprising a plurality of combined cylindrical bodies which are disposed in a cylindrical vacuum chamber. Outside of the vacuum chamber two annular permanent magnets are arranged so as to sandwich the vacuum chamber therebetween. Each of the permanent magnets has a shape corresponding to the annular cathode electrode and the annular anode electrode.
With these conventional sputter ion pumps, two annular permanent magnets are arranged to sandwich the annular anode electrode, significant magnetic field is formed in parallel to the central axis. With regard to radial magnetic field perpendicular to the central axis, the radial magnet field on the central axis becomes zero if said two annular permanent magnets have same size and same characteristic. Significant large magnetic field is formed in an area where is near to the central axis (for example, the position of 0.5 to 1 mm from the central a). However, the radial magnet field on the central axis does not become zero but is significantly large due to an unevenness in the characteristics of the magnets.
The arrangement disclosed in Japanese UM Publication No. 3-48838 has a disadvantage that the pump itself has a heavy weight due to the provision of a yoke circuit.
In addition, there is another disadvantage that the leakage magnetic field is larger to have an adverse effect to a beam deflection. That is, if the leakage magnetic field becomes larger, electron beams in the accelerator or the electron microscopy are to be deflected and thus an electron image becomes dim and a current value of the electron beam is reduced. In particular, with the arrangement disclosed in the above mentioned Japanese Patent Publication No. 7-59943 that no yoke member is provided, the magnetic field induced from the permanent magnetic field has an adverse effect to neighboring measuring instruments.
Furthermore, in view of a pump performance it is important that the surface area of the respective members or parts built in a vacuum chamber is as small as possible for attaining an ultra-high vacuum (UHV). In the conventional sputter ion pump mentioned above, however, the cathode electrode and the inner wall of the vacuum chamber have relatively larger surface area, and the amount of gas discharged from these portions is relatively larger. Therefore, the ultimate pressure of a pump is to be restricted
Therefore, the present invention has an object to overcome the above mention disadvantages involved in the prior arts and to provide a sputter ion pump that it has a simple structure, a size and a weight can be reduced, the magnetic fields in the vicinity of a central axis can be nullified in the both of a radial and axial directions, and ultimate pressure of the pump can be increased.
In order to attain the above mentioned object, according to the sputter ion pump of the present invention a vacuum chamber includes an inner wall having a tubular or annular section where is formed to be rugged in a cross section to define outer recesses and inner recesses. The outer recesses of the rugged tubular section are intended to receive permanent magnets each of which has a same shape and a same characteristic and arranged so that a magnetic pole is directed to a same direction. The inner recesses are provided with anode electrode members spaced from the vacuum chamber wall. The rugged tubular section of the vacuum chamber wall is formed as a cathode electrode. A cylindrical shield member having a peripheral portion provided with evacuating bores is provided coaxially to the permanent magnets and anode electrode members, the permanent magnets and anode electrodes being arranged with equal spacing in a configuration symmetrical against the center axis.
Each of the permanent magnets arranged in the respective outer recesses of the tubular section may be formed as a wedge-shaped polygonal or circular column having an outwardly spread configuration in a cross section perpendicular to the central axis of the vacuum chamber.
Each of the anode electrode members arranged in the respective inner recesses of the cylindrical section may be formed as a wedge-shaped circular or polygonal tubular body having an outwardly spread configuration in a cross section in a direction perpendicular to the central axis of the vacuum chamber.
The outer recesses and the inner recesses of the cylindrical section of the vacuum chamber may be arranged alternately, and the permanent magnets and the anode electrode members may be arranged alternately.
Preferably, the peripheral portion of the vacuum chamber having the recesses in which the permanent magnets and the anode electrode members are arranged and the magnetic shield member may be cylindrical. The permanent magnets and the anode electrode members may be arranged symmetrically against the center axis on a substantial same circumference.